Gas turbines include a compressor, a combustion chamber and a turbine to expand the hot gas generated in the combustion chamber.
In addition, the gas turbines can have a second combustion chamber downstream of the turbine and a second turbine downstream of the second combustion chamber.
The combustion chamber and second combustion chamber (when provided) are supplied with air from the compressor and fuel; the fuel is combusted to generate the hot gas.
When the fuel is natural gas, its composition and features can largely change over time. For example, the amount of nitrogen (inert component because it does not take part in the combustion process) can largely change over time.
Since the total energy provided into the combustion chambers (i.e. the Lower Calorific Heating Value of the fuel injected into the combustion chamber) defines the load of the gas turbines, the fuel mass flow injected into the combustion chamber can largely vary when the features of the fuel vary also if the gas turbine load is kept constant or substantially constant.
When the fuel is injected into the combustion chamber, this large change in fuel mass flow caused by the change of fuel composition can cause a change in the fuel jet penetration into the combustion chamber and, thus, poor mixing, large emissions such as NOx emissions and reduced efficiency.